1. Field of the Invention
The invention relates generally to threaded fasteners and fastener drives for the fasteners which make up a fastener drive system. The fastener drive system of this invention is especially useful in the medical arts which require that the fastener be oriented on the end of a drive tool in a coaxial position to eliminate any angular excursions between a prosthesis, the bone and the fastener.
2. Description of the Prior Art
There is a great deal of prior art teaching various forms of drive systems where one component is termed female and the other component is termed male with a cooperating recess and projection, respectively. Probably, the most common is the slotted screw head formed to accept the blade of a screw driver. Almost everyone knows that the tolerance between the slot and the blade is such that small side loads will turn the screw and disconnect the screw driver thereby creating a bore in the support that is larger than the thread diameter of the fastener. Further, as the screw tightens to form a fastening, the increasing torque causes the blade to spin out of the slot.
In surgery, the operating field is as small as possible and usually very confined which may restrict the approach of various tools to a less than optimum angle to the work area, producing side loads and reduced visibility, among other things, when placing a threaded fastener. The skeletal bone is made up of a hard outer casing surrounding a softer inner tissue so that maximum fastening forces are located in the outer thickness of the bone. Any wobble of the alignment of the fastener can lessen the purchase of the threaded fastener with the bone and significantly reduce the holding power of a threaded fastener.
U.S. Pat. No. 5,019,080 to Hermer teaches a drive system and fastener for surgical applications. The driver imparts rotational torque to the fastener and includes a tapered drive bit engagable with a tapered socket formed in the fastener. The bit and socket are formed with cooperating hexalobular surfaces. These surfaces form a triangular contact between the driver and the socket to prevent wobbling of the fastener when driven by the driver.
U.S. Pat. No. 5,279,190 to Goss et al is directed to an elliptical lobed drive system of general utility having a hexagonal configuration. One of the components, either the externally configured or the internally configured flutes and lobes, will be generated from ellipses of substantially equal dimension whereas the other component has flutes and lobes generated by ellipses of differing dimensions. The cooperating surfaces form a tapered interconnection.
The patent discusses the vector analysis of the forces generated during rotation of the system as composed of two components, a radial vector and a tangential vector. The tangential component is said to rotate or drive the fastener and is termed, “drive angle,” which is defined by the angle made by a line tangent to the point of driver contact at the point of application and a radial line through the fastener or drive tool. The patent also states that generally speaking, the lower the drive angle, the more efficient the drive system. Further, when the drive angle exceeds a certain value, as for example 60 degrees, the torque loss is excessive and this situation should be avoided. Significantly, the more efficient the drive system, the less depth required for engagement of the lobes and flutes permitting smaller tools and screw heads on the fasteners.
By using the hexalobular construction in the drive system, these prior art devices rely on establishing a triangular contact array during operation of the system. In addition, the socket has a significant depth required by the triangular contact surfaces. However, the tapered form of the interconnection produces a tendency for the driving tool to spin-out of the recess under heavy torque forces.
What is needed in the art is a system to increase the contact surfaces and eliminate stress risers thereby reducing the force applied at each contact point, reducing the depth of the interconnection, shaping the walls of the interconnection to reduce spin-out and increasing the stability of the coaxial alignment between driver and fastener during rotation.